1. Field of the Invention
This invention relates generally to photographic cameras and, more particularly, to real-image viewfinders for photographic cameras.
2. Description of the Related Art
It is desirable for a camera to provide a viewfinder device that permits a photographer to view an object whose image is to be recorded by the camera. In a photographic camera with through-the-lens viewing, a real image of the object is formed at a focusing screen and is reflected through the viewfinder to the photographer. A mirror is located in the exposure optical path to reflect the object image to the focusing screen and permit viewing the image of the object, and is moved out of the exposure path when the film shutter is opened. A through-the-lens viewing system thereby permits the photographer to see actual changes in focus and magnification of the object image up to the instant of exposure, but unfortunately such viewing systems can be heavy, complex, and relatively expensive to produce.
Savings in weight, complexity, and cost can be realized by eliminating the through-the-lens viewing system and providing a viewing system having an optical path separate from the exposure path. Viewfinders of the Galilean type provide a real image of the object to be photographed and can be adapted to permit changing the viewfinder focus, with which the photographer can see changes in focus of the camera lens, and to permit continuously changing the viewfinder field of view, which is commonly referred to as a zoom function. Such a viewfinder system is described, for example, in U.S. Pat. No. 5,155,517 entitled Real Image Zoom Viewfinder, of which the present applicant is a co-inventor.
A telescopic viewfinder generally includes at least an objective lens of positive focal length located nearest an object to be photographed, an eyepiece lens through which the photographer views the real image, and a reflecting optical unit that erects and reverses the image of the object. The objective lens may be a zoom lens of variable focal length. The real image is formed in the viewfinder optical path between the objective lens and the eyepiece lens. If the objective lens is a zoom lens including a variator but no compensator, then the location of the real image is shifted along the length of the optical path as the focal length and field of view are changed.
It is desirable for a camera viewfinder to provide information in the viewfinder in addition to the object image. For example, photographers prefer a viewfinder display that includes a central focus aid and that shows information such as shutter speed, aperture setting, exposure, focus information, and the like. In a telescopic viewfinder, such information can be viewed by the photographer if a display is placed at the plane of the real image. Reading the display at the real image plane is difficult, however, if the position of the real image is shifted as the focal plane position of the zoom objective lens and field of view are changed. For a simple zoom objective lens without compensation for focal plane shift, the real image will drift through the focal plane of the eyepiece lens while the display remains stationary at the eyepiece lens focal plane, or if compensation is provided by moving reflective surfaces to position the focal plane formed by the objective lens at a constant distance from the eyepiece lens, the display must be adapted to move with the real image plane to stay in the focal plane of the eyepiece lens.
Another difficulty encountered with the display of information in a real image viewfinder is attempting to maintain a constant ratio of illumination of the scene to illumination of the information. The displayed information should be sufficiently bright to be easily read by the photographer. If the information is illuminated with an auxiliary light, however, it can be difficult to read because of the disparity in the light level between the object being viewed under a variety of ambient lighting conditions and the display being illuminated at a constant level of brightness.
From the discussion above, it should be apparent that there is a need for a viewfinder that provides a real image of the object to be photographed and displays information in the viewfinder that easily can be read by a photographer as the viewfinder objective lens focus and field of view are changed and as scene brightness varies. The present invention satisfies this need.